1. Field of the Invention
The invention relates to an optical mouse having an image capture chip, and more particularly to an image capture chip capable of effectively adjusting an exposure value.
2. Description of Related Art
Recently, image capture chips are widely used in various digital devices such as digital cameras, video cameras, and optical pointing devices. Image capture chips play an important role in capturing a projected image of an object for subsequent related digital image processing. Based on a theory of exposure, the image capture chip adjusts an exposure parameter according to an illumination of the object, thereby determining a suitable exposure duration.
Thus, by using the adjusted exposure parameter, the corresponding average exposure value thereof is able to be converged within a normal-exposure range, even if the illumination of the object is too dark or too bright.
FIG. 1A is a diagram of a conventional image capture chip. Each of the pixel units PIX11˜PIXnm is a pixel circuit of the image sensor 100. The image capture chip 10, initially, reads captured image data line by line and stores the pixel signals of the pixel units PIX11˜PIXnm via a signal reading circuit 130. The pixel signals of the signal reading circuit 130 are amplified by an amplifier 150. Finally, the A/D converter 170 converters the amplified pixel signals into a plurality of digital pixels. Thus, a digital signal processor (DSP) 11 accordingly performs related image processing such as calculating the average exposure value or displaying the captured image.
FIG. 1B is a diagram of the image sensor 100 of FIG. 1A. If a CMOS M2 is turned on according to a reset signal RES, the potential across an optical diode PD is charged to a voltage Vrst. The voltage Vrst decreases as the optical diode PD is exposed under the illumination of the image. Each row of sensor units PIX1m˜PIXnm as shown in FIG. 1A comprises its storage 131. When a NMOS M3 is turned on by a select signal RSEL2, the residual voltage across the optical diode PD is transformed into a current across an NMOS M1. The current charges a corresponding storage 131 of a signal reading circuit 130. The charge of each storage 131 is referred to as an illumination value of the corresponding pixel unit.
The illumination intensity and the illumination duration of the optical diode PD are referred to as an exposure intensity and an exposure duration of the image sensor 100, respectively. Based on the theory “Exposure value=Exposure intensity*Exposure duration”, the exposure value can be adjusted if the exposure duration or the illumination duration is changed. Hence, to maintain the exposure quantity, the exposure duration is decreased when the exposure intensity is increased. Alternatively, the exposure duration is increased when the exposure intensity is decreased.
Generally, an exposure parameter is provided for adjusting the exposure duration. For example, to increase the exposure value according to the conventional exposure control method, the exposure parameter is linearly added with a predetermined exposure parameter. Alternatively, for decrease the exposure value, the exposure parameter is linearly subtracted by a predetermined exposure parameter.
It is difficult, however, to define the predetermined exposure parameter of the conventional exposure control method. For example, if the exposure parameter is adjusted with an excessively large predetermined exposure parameter, the illumination of a digital image is substantially changed. Thus, it is difficult to converge the corresponding exposure value into a normal-exposure range. Similarly, if the exposure parameter is adjusted by an excessively small predetermined exposure parameter, the illumination of the object is changed slightly. Thus, numerous adjustment steps are only required, resulting in slow convergence. Hence, the digital image is excessive dark or excessive bright due to a long exposure duration, thus the image quality is adversely affected.